PDA

View Full Version : Gedmatch “X-one-to-one” matches question



Power77
03-22-2018, 04:15 PM
Hello everyone,

I logged into my Gedmatch account this morning and found out (or rather noticed) that I have ~1950 matches on my Gedmatch “X-one-to-one” account. That said, am I not supposed to have way less X-DNA matches as a man (as I only have one X chromosome which I got from my mother)? Or is it possible for men to have that many matches on the X?

Any good answers and/or explanations are welcome;)!

geebee
03-23-2018, 07:01 AM
I suspect the chief answer lies in endogamy. Coming from Ashkenazi and Sephardi populations, you likely have a smaller number of distinct ancestors relative to present population size. Therefore, more people share segments with you -- including segments on the X chromosome.

But even apart from endogamy, every person has a smaller pool of possible X-chromosome contributors. To illustrate, consider your maximum number of ancestors from 10 generations before you. It's 2^10, or 1,024. This doesn't take into consideration pedigree collapse, which is actually quite likely. But it will serve to illustrate the point.

The maximum number who can have contributed to your X chromosome in this generation is only 89! (See https://thegeneticgenealogist.com/2008/12/21/unlocking-the-genealogical-secrets-of-the-x-chromosome/) For a woman this number is higher -- 144 -- but it's still only a fraction of the total number of ancestors.

So, again, the key is likely endogamy, plus the fact that there are fewer distinct X chromosomes (and therefore X chromosome segments) to go around.

Power77
03-23-2018, 11:35 AM
I suspect the chief answer lies in endogamy. Coming from Ashkenazi and Sephardi populations, you likely have a smaller number of distinct ancestors relative to present population size. Therefore, more people share segments with you -- including segments on the X chromosome.

But even apart from endogamy, every person has a smaller pool of possible X-chromosome contributors. To illustrate, consider your maximum number of ancestors from 10 generations before you. It's 2^10, or 1,024. This doesn't take into consideration pedigree collapse, which is actually quite likely. But it will serve to illustrate the point.

The maximum number who can have contributed to your X chromosome in this generation is only 89! (See https://thegeneticgenealogist.com/2008/12/21/unlocking-the-genealogical-secrets-of-the-x-chromosome/) For a woman this number is higher -- 144 -- but it's still only a fraction of the total number of ancestors.

So, again, the key is likely endogamy, plus the fact that there are fewer distinct X chromosomes (and therefore X chromosome segments) to go around.

Thanks for your answer Geebee:). I’d like to mention the fact that I don’t get so many matches beyond 7 cMs. Why is that:confused:? Also, have you seen male Gedmatch kits with ~1950-2000 X-DNA matches at 7 (or less) cMs?

geebee
03-23-2018, 04:12 PM
Thanks for your answer Geebee:). I’d like to mention the fact that I don’t get so many matches beyond 7 cMs. Why is that:confused:? Also, have you seen male Gedmatch kits with ~1950-2000 X-DNA matches at 7 (or less) cMs?

No, I'd have to say that this seems a very large number of X matches for anyone of either sex. But, I am aware that people with Ashkenazi ancestry often do have an unusually large number of segment matches throughout the genome. This is a common enough phenomenon that some companies have developed "corrections" for Ashkenazi descendants, as an attempt to deal with this "excess matching".

I believe it simply comes from there having been a relatively small population in the past that was more included to marry within itself than outside, for multiple reasons. I don't know much about either the publication (Haaretz) or the article, but you might find this to be of some interest:

https://thegeneticgenealogist.com/2008/12/21/unlocking-the-genealogical-secrets-of-the-x-chromosome/

Even if the numbers themselves are exaggerated -- say, perhaps, that the initial population was not quite as low as the article suggests -- the point is still than there was a fairly small starting population. So there would only have been so much distinct DNA to go around, and all current members of the population would still show a high degree of relatedness.

(Of course, every human is related to every other human, and most of our DNA is actually the same. It's just that the testing companies are only focused on where we do have some differences.)

Power77
03-23-2018, 06:50 PM
No.

Perhaps it’s due to the fact that you have yet to take a look at Jewish Gedmatch kits;).